An auditory event boundary detector, according to aspects of the present invention, processes a stream of digital audio samples to register the times at which there is an auditory event boundary. Auditory event boundaries of interest may include abrupt increases in level (such as the onset of sounds or musical instruments) and changes in spectral balance (such as pitch changes and changes in timbre). Detecting such event boundaries provides a stream of auditory event boundaries, each having a time of occurrence with respect to the audio signal from which they are derived. Such a stream of auditory event boundaries may be useful for various purposes including controlling the processing of the audio signal with minimal audible artifacts. For example, certain changes in processing of the audio signal may be allowed only at or near auditory event boundaries. Examples of processing that may benefit from restricting processing to the time at or near auditory event boundaries may include dynamic range control, loudness control, dynamic equalization, and active matrixing, such as active matrixing used in upmixing or downmixing audio channels. One or more of the following applications and patents relate to such examples and each of them is hereby incorporated by reference in their entirety:    U.S. Pat. No. 7,508,947, Mar. 24, 2009, “Method for Combining Signals Using Auditory Scene Analysis,” Michael John Smithers. Also published as WO 2006/019719 A1, Feb. 23, 2006.    U.S. patent application Ser. No. 11/999,159, Dec. 3, 2007, “Channel Reconfiguration with Side Information,” Seefeldt, et al. Also published as WO 2006/132857, Dec. 14, 2006.    U.S. patent application Ser. No. 11/989,974, Feb. 1, 2008, “Controlling Spacial Audio Coding Parameters as a Function of Auditory Events,” Seefeldt, et al. Also published as WO 2007/016107, Feb. 8, 2007.    U.S. patent application Ser. No. 12/226,698, Oct. 24, 2008, “Audio Gain Control Using Specific-Loudness-Based Auditory Event Detection,” Crockett, et al. Also published as WO 2007/127023, Nov. 8, 2007.    International Application under the Patent Cooperation Treaty Serial No. PCT/US2008/008592, Jul. 11, 2008, “Audio Processing Using Auditory Scene Analysis and Spectral Skewness,” Smithers, et al. Published as WO 2009/011827, Jan. 1, 2009.
Alternatively, certain changes in processing of the audio signal may be allowed only between auditory event boundaries. Examples of processing that may benefit from restricting processing to the time between adjacent auditory event boundaries may include time scaling and pitch shifting. The following application relates to such examples and it is hereby incorporated by reference in its entirety:    U.S. patent application Ser. No. 10/474,387, Oct. 7, 2003, “High Quality Time Scaling and Pitch-Scaling of Audio Signals,”, Brett Graham Crockett. Also published as WO 2002/084645, Oct. 24, 2002.
Auditory event boundaries may also be useful in time aligning or identifying multiple audio channels. The following applications relate to such examples and it are hereby incorporated by reference in their entirety:    U.S. Pat. No. 7,283,954, Oct. 16, 2007, “Comparing Audio Using Characterizations Based on Auditory Events,” Crockett, et al. Also published as WO 2002/097790, Dec. 5, 2002.    U.S. Pat. No. 7,461,002, Dec. 2, 2008, “Method for Time Aligning Audio Signals Using Characterizations Based on Auditory Events,” Crockett, et al. Also published as WO 2002/097791, Dec. 5, 2002.
The present invention is directed to transforming a digital audio signal into a related stream of auditory event boundaries. Such a stream of auditory event boundaries related to an audio signal may be useful for any of the above purposes or for other purposes.